A primary objective of population genetics is to determine the nature of genetic variation and how it is maintained in natural populations. Mimulus guttatus is a wildflower that exhibits substantial genetic variation in flower size, a trait that has evolved extensively within the species complex. We will map the genetic loci responsible for variation in flower size within a single natural population of M. guttatus and determine the magnitude and distribution of allelic effects on a range of plant characters. The experiments will estimate the extent and pattern of dominance at QTL and the pleiotropic effects associated with flower size alleles. A streamlined breeding scheme will produce Nearly Isogenic Lines (NILs), necessary for positional cloning of the actual loci defining important QTL. They will also be used in field and lab studies to measure the direct fitness consequences of flower size variation at the scale of individual QTL. These data will be used to test hypotheses regarding the selective maintenance of flower size variation. Combined with results from previous and ongoing studies, these experiments will provide a detailed comparison of inter- and intra- specific variation in floral morphology within the M. guttatus species complex. Positional cloning of important floral QTLs will allow molecular population genetic studies of ecologically important genes. The proposed research will have broader impacts thorough both education and the development of scientific resources.